Four seasons ago a quartet of young women began receiving threatening, and very well informed texts, from the mysterious "A," who they assumed was their former queen bee Alison, presumed dead. So you can imagine their shock. Now, of course, they know Alison is alive and well, as is "Pretty Little Liars." It quickly became ABC Family's break-out hit and the most tweeted about series in television, setting a template now followed by every other show, including, most recently, the Oscars.

So if Alison's not "A," who is? For years, the Liars and their fans have tried to figure it out, and while theories turned and turned again (if it's not Ezra could it be Alison's mother after all?), it seems unlikely the answer will be revealed in the March 17 season finale. And since solving the mystery could end the show, we really don't want to know.

Who do you think A is? And why do you think "Pretty Little Liars" has kept us guessing all these years? Join TV reporter Yvonne Villarreal, TV critic Mary McNamara and business reporter Dawn Chmielewski at noon on Monday to discuss how "Pretty Little Liars" is able to do what it does so well. Tweet comments to #askLATimes.